Environmental impacts of sea-level rise and

associated adaptive strategies

Katie Jewitt

Outline

• Environmental impacts of sea-level rise• Coastal ecosystems and their services• Environmental impacts of coastal protection strategies• Criteria for environmental vulnerability and design

considerations for environmental impact assessment• guidelines on choosing the preferred strategy

Environmental impacts of sea-level rise• increases erosion of coastline• inundation of low-lying lands• loss of wetlands due to increased erosion and flooding• increased flooding and storm-damage to low-lying

coastal lands• salinity intrusion

Erosion

bigger wave base, as well as stronger storm intensity

Inundation

 

http://seagrant.gso.uri.edu/newsletter/mar_apr08/coastalcommunities.html

 

 Loss of saltmarsh area (% loss from 2000 baseline), under the IPCC low sea-level rise scenario, 2080

Loss of saltmarsh area (% loss from 2000 baseline), under the IPCC high sea-level rise scenario, 

http://www.branchproject.org/achieve/wildlifemapping/coastalvulnerability/

 

 

1 meter of sea level riseputs South Louisiana under water.

http://healthygulf.org/blog/labels/Global%20Warming.html

Wetland loss

Most coastal wetlands in the mid-Atlantic would be lost if sea level rises one meter in the next century. Even a 50-cm rise would threaten most wetlands along the Eastern Shore of Chesapeake Bay. 

Salinity intrusion

- threatens freshwater supply

- threatens estuarine fisheries and certain seafood species (e.g. oyster)

en.wikipedia.org/wiki/North_Sea

Ecosystem services: Beach and dunes

Habitat

http://forums.miamibeach411.com/index.php?ACT=35&fid=39&aid=395_873D9dRDRfRT8H5mQEqHhttp://www.hedweb.com/animimag/turtles.jpg

http://www.longboat-key-florida.info/images/birds-beach-348x278.jpg

• Habitat• Nutrient uptake• food production• wave attenuation• sediment stabilization: dependent on the presence

of vegetation• raw materials: 

  sand and gravel

http://www.fao.org/docrep/010/ag127e/AG127E09.htm

Ecosystem services: Beach and dunes

Ecosystem services of marshes, mudflats and

other vegetated communities• Habitat: support large local fisheries

• Nutrient uptake• Food production: macroalgae, detritus,

migratory birds• Wave attenuation• Sediment  stabilization• Maintenance 

of biodiversity• Production 

 of raw materials

 

 

Wave attenuation by mangrove forest (Rhizophora sp., Aegiceras sp., Ceriops sp.) at Cocoa Creek, Australia is obvious; measurements at sites 2–5 show the decline in wave energy transmission through the mangrove forest. The incoming wave was measured at site 1 (Massel et al.,1999)

 

 

Coastal erosion sites reported in Asian and Indian Ocean countries; the inset indicates how clearing of coastal forest such as mangroves has increased the vulnerability of coasts to erosion (base map source from ITDB, 2004)www.fao.org/docrep/ 010/ag127e/AG127E86.jpg

Adaptation strategies

Adaptation strategies

• Hard structures:• Wetland loss• Influence banks, channels, beach

profiles, sediment transport and morphology

• Soft structures: retain natural coastlines, but dredging may cause disruption

• Some potential benefits: artificial reefs may create new habitats, dams may mitigate salinity intrusion

Seawalls

 

Waves pounding the sea wall at Walcott, UK, November 9 2007. Photo: John Giles / AP

 

http://safecoast.org/actueelarchief/?actie=weergeven&weergeef=alles

Impacts of Seawalls

• Impacts on beaches:o Formation of a scour trougho Formation of a deflated profile : the uniform general

lowering of the fronting beacho Formation of beach cusps : semi-circular, seawards

opening embaymentso Formation of a rip current trough : a linear shore

normal depressiono Terminal scour : accelerated active erosion on

beaches and coasts immediately down-drifto Up-drift sand accretion due to impounding at the up-

drift end of the wallThis is what happens to the beaches if seawalls are erected. To protect against toe scour and seawall failure, more and more armament must be added as you can see with the multiple layers installed to protect the Galveston seawall.

Rotational currents moving off the 90º angle of the seawall cause accelerated erosion to adjacent properties.

Impacts of Seawalls

• Impacts on beaches:o Formation of a scour trougho Formation of a deflated profile : the uniform general

lowering of the fronting beacho Formation of a rip current trough : a linear shore

normal depressiono Terminal scour : accelerated active erosion on

beaches and coasts immediately down-drifto Up-drift sand accretion due to impounding at the

up-drift end of the wall o Formation of beach cusps : semi-circular, seawards

opening embayments

1) Sea walls destroy eroding beaches first by reducing the size of the beach.

2) As water moves in, it eventually meets the wall, flooding the beach.

3) The wave action causes the underlying sand to erode quickly, undermining the wall.

Impacts of Seawalls

• Impacts on beaches:o Formation of a scour trougho Formation of a deflated profile : the uniform general

lowering of the fronting beacho Formation of a rip current trough : a linear shore

normal depressiono Terminal scour : accelerated active erosion on

beaches and coasts immediately down-drifto Up-drift sand accretion due to impounding at the

up-drift end of the wall o Formation of beach cusps : semi-circular, seawards

opening embaymentsEccles, UK. Photo: Mike Page, Marinet

 

 

Coastal protection efforts to protect a valuable tourism base; meanwhile, the adjacent shore with less economic value has minimal and improper protection. Even revegetation with waru to replicate planting at the neighbouring resort failed; the coast was then abandoned and left to erodeSource: http://www.fao.org/docrep/010/ag127e/AG127E87.jpg

Increasing the success of seawalls

• a wall which cuts off an area's only supply of sediment (e.g. a cliff) will have a major impact on the beach, while an area which has many sources of sediment (e.g. from offshore and longshore drift) will have alternate sources and will not have to rely on the fronting beach

• wave energy will dissipate naturally over beach profiles, and so the further inland sea walls can be placed, the fewer problems with interference with incident waves, and the less seaward penetration into the surf zone

• walls which dissipates energy, by absorption or by random deflection on an irregular surface; or focus wave energy on revetment

• nourish fronting beach

Groins

Groin field

Groins and jetties

• Sediment accretion in the updrift side, sediment erosion in the downdrift side

• Narrowing of beach on downdrift side

• Interruption of longshore transport• modification of channel processes

o interaction of jetty and river mouth dynamics critical in the  functioning of the sediment supply and transport processes

Mitigation: pumping of sediment to transfer material mechanically around jetties

Dikes and Levees

• May have similar impacts as emergent breakwaters on coastal hydrodynamics, sediment transport and geomorphology

• exclude natural dynamics (regular flooding) from the diked marshlands

• Borrow area

 

This borrow area is graded to drain and is planted in trees restoring bottomland hardwoods and terrestrial wildlife habitat

This borrow area is irregularly shaped with smooth side slopes, varying depths and islands with trees left undisturbed in the middle.  This type design promotes fisheries and waterfowl benefits. 

Source: Mississippi Levee Board

 • exclude naturally wide brackish water

transition zones with various habitats• Impacts very site-specific• Cases where built in estuaries and

rivers: ochannelization of rivers and tidal

channelsowetlands may not be able to keep up

with sea level riseo loss of wetlands

• Reduction of intertidal    habitat

Revetments and bulkheads

http://www.snh.org.uk/publications/online/heritagemanagement/erosion

Revetments and Bulkheads

• Reflects wave energy, eroding coastline elsewhere

• Loss of intertidal habitato For a 1m rise, 29-66% coastal wetland

loss with retreat, but 50-82% if protected with bulkheads

• Increase in the rate of lowering of fronting beach

Breakwaters

• sediment buildup in lee•  reduction in erosion rates and impacts on sediment budget

-> erosion elsewhere• decreased wave activity and impacts on supratidal

vegetationo reduction of salt sprayo invasion of non-salt tolerant species -> loss of rare

habitats• isolation of foreshore from active coastal environment

o may reduce oxygen levels, exacerbated by pollution buildup

o may reduce circulation of water -> reduce water mixing, decrease flushing times -> pollutant buildup

Breakwater -- Port in isolated environment

• sea without tide and very little river contribution if the coast is rocky, erosion is negligible if the coast is sandy, erosion is important but

will be limited in space if there is no coastal current

Breakwater -- Port at rivermouth

• blocks coastal sediment transport• coast attacked hard by wave refracting around breakwater --

increased erosion• stability issue

Source: Coastal Wiki

Breakwater -- Port at mouth of large estuary

• Naturally, mudflats and wetlands may migrate downstream due to river flow

• breakwater canalise river, preventing sedimentation, and therefore migration of the mudflat areas toward downstream -> loss of valuable mudflats

Breakwaters -- potential benefits

• new habitat for kelp, marsh and seagrasses• (if built of stone) provide hard substrates beneficial to algae,

barnacles and oysters• creates foraging area for fish• habitat for fishes, higher fish species richness than natural

reefs(?) (but other study suggested lower observed total diversity) 

• trade-off when a natural habitat is replaced by a man-made structure

Beach Nourishment

Qualitative relationship between upland economic base and long-term erosion rate. (From Dean and Dalrymple, 2002)

Sand transport losses and beach profiles associated with a beach nourishment project.

Source: http://www.csc.noaa.gov/beachnourishment/html/geo/scitech.htm

Beach Nourishment

• burial of shallow reefs and invertebrates• reduce food availability for birds, fishes

and crabs• replacement of habitat from nearshore

benthic community to an intertidal and supratidal beach and dune

Beach Nourishment: potential environmental

benefits• Increased habitat for sea turtle nesting, • nesting and foraging areas for sea birds, • Habitat for beach flora, e.g. sea beach

amaranth and bitter panica

Sea Beach Amaranth (Newsday File photo)

Marked turtle nest and unusual single exposed egg (right foreground) on an eroding Florida beach.Source: NBII

Marsh building

• Beneficial in terms of habitat• increased wildlife potential for

estuaries• useful way of using unwanted

dredgings for    increased coastal    defense

Design consideration for environmental impact assessment• Geographical characteristics

o location: whether port is at mouth of estuary, river outlet, or in an isolated environment

• Physical characteristicso wave and tide characteristicso sediment budget and transport

• Biological characteristicso need to maintain coastal wetlands?

identify key ecosystems in surrounding areas

- geographical characteristics:-- - biodiversitykey specieseffects of nutrient fluxeseffects on larval dispersal and recruitment

Guidelines to choosing adaptation strategy options

1. Overview of justification of requirements of coastal defense2. Data gathering

1.Collection of data on physical processes and characteristics• Geology and geotechnics• Nearshore seabed

1.     Bathymetry2.     Seabed sediments1. Waves, winds and tides2. Coastal defenses (the standard of defense and residual life of

existing coastal structures1.Data on benefits and costs2.Data on environmental constraints and opportunities

Biological, Physical, Socio-economic, Aesthetic, Chemical

3. Assessing the effect of not intervening

1.Assessing hydrodynamics and sediment transport rates• Wave conditions, at a location just offshore from surf zone• Tidal levels including the effects of surges• Information on the joint probability of large waves and high

tidal levels to provide estimates of the conditions that any defense may encounter1.Assessing existing defenses2.Predicting future changes in the coastline and standards of

defense3.Preliminary economic evaluation4.Preliminary environmental appraisal

 

1.Public consultation2.Assessing coastal defense options

Conclusions

• adaptation methods all have their own environmental pros and cons, with hard structures typically being detrimental to the environment

• a combination of different structures (hard and soft) is often beneficial

• important to evaluate each site case-by-case basis to select the best environmental procedure

• detailed environmental appraisal and data analysis is necessary for preparation

• important to look at the coastal system as a whole, integrative manner, rather than tackle the SLR problem in a piecemeal fashion